This invention relates to a method for measuring the state of a fluid and, more particularly, to a fluid state measuring method capable of collectively measuring a change in state of a stagnant or running fluid on an in-line processing basis.
The term "fluid" as used in the present specification means all kinds of fluid, i.e., a gaseous substance, a liquid substance or a solid substance such as powder, or of two or more of these substances, as well as a fluid of the type whose phase changes with time, and wherein a "change in state" of the fluid is represented by a change in the viscosity, density, specific heat, thermal conductivity, thermal diffusivity, coefficient of volumetric expansion, flow speed or flow direction of the fluid.
Direct measurement of the viscosity, density and other parameters is very important for process control in a variety of industrial fields that handle fluids, and is especially indispensable for process automation. For determining fluid viscosity, which is one of the most important indices representative of the state of the fluid, a rotational viscometer is most widely used. With the rotational method, the force or torque required for a cylinder to rotate at a constant rate in the fluid is measured in a conventional manner. Another method is based on measurement of the pressure difference between an inlet port and outlet port of a narrow pipe when a predetermined amount of fluid which is practically free of any solids matter such as dust passes through this pipe. However, these mechanical methods have been practised in only limited industrial, medical or academical fields, and their widespread applications to general in-line process control or measurement for fluids has not been realized. This is firstly because the structure of semi-solid fluids such as food gels is destroyed when these mechanical measurements are performed, and secondly that highly viscous fluids display a rate-dependent behavior such that the mechanical methods may only be applied to clean, low viscosity fluids.
In practical processes or measurements, however, such a clean, low viscosity fluid is exceptional and there are a variety of fluids, including liquid systems, which contain highly viscous slurries, printing inks or food gels, whose apparent viscosity is decreased by stirring. When a system contains bubbles whose diameter changes with time, or when a gaseous system contains water droplets in suspension, or when a liquid system contains a colloid such as oil droplets and/or small matter like colloidal metal particles, the conventional methods are not usable. Therefore, the direct measurement of fluid viscosity, or of a collective and characteristic fluidity value representative of viscosity, for the purpose of automatic process control in a practical industrial process has been difficult to achieve.
New methods for measuring fluid viscosity have recently been proposed which are different in principle from the conventional mechanical methods. One method appearing in Japanese patent application Laid-open No. 59-217162 (U.S. Pat. No. 4,611,928) can examine the curd state of milk during a milk-curdling process by inserting a thin metal wire into the milk and measuring the temperature of this thin metal wire in relation to time while applying an electric current intermittently or continuously to the thin metal wire. This method is based on the principle of detecting milk curdling as a function of the change in viscosity of the milk on the basis of a temperature rise in the electrically heated metal wire. In another method proposed in Japanese patent application Laid-open No. 60-152943 (U.S. Pat. No. 4,578,988), a change in the property of a liquid or semi-solid substance is measured by inserting a thin metal wire into the said substance and applying an electric current intermittently or continuously to the thin metal wire. The difference in temperature between the hot metal wire and the fluid surrounding the wire is kept constant with time. The value of the electric current to maintain that constant temperature difference is measured, and a coefficient of heat-transfer at the surface of the wire is calculated on the basis of the measured electric current, the electrical resistance of the wire, the surface area of the wire and the temperature difference kept at a constant known value. Thus, this method for calculating a characteristic value of the fluid is based on the amount of electric current applied to the wire.
These proposals are, however, directed to measuring the viscosity of a specific fluid such as stagnant or substantially stagnant liquid, whose composition and phase do not change, and their widespread applications to general process control and academic measurements for various kinds of fluid cannot be realized without difficulties. Furthermore, in many applications of actual process control, the in-line measurement of quantities which collectively reflect the state of a fluid may depend on all the parameters of viscosity, density, specific heat, thermal conductivity, thermal diffusivity, coefficient of volumetric expansion, flow speed or flow direction of the fluid, etc. These quantities may be of more significance for accurate measurement than a specified characteristic value indicative of viscosity only. Therefore, the conventional mechanical methods are totally unsuited for such collective measurements.